The present invention relates to pistons for hydraulic pumps and motors. More particularly, this invention relates to a filling for hydraulic pistons used in pumps and motors. The filled piston of this invention increases the efficiency of the pump or motor at a reasonable cost.
A known technique for reducing the amount of oil that is contained within a hydraulic piston is to fill the normally hollow piston with a solid material. This reduces the amount of oil contained within the piston. The oil within the piston must be compressed during each revolution or pumping cycle.
Hollow piston constructions have been found to produce adverse side effects due mainly to the compressibility of the oil which fills the piston cavity. The compressibility of the fluid has a marked effect upon the overall efficiency of the unit, and also produces cavitation, erosion, noise and undesirable moments on the swashplate mechanism when used in an axial piston type of pump or motor.
There are currently at least three known types of "filled" hollow pistons: welded pistons, solid pistons, and plastic-filled pistons. Welded pistons are costly to manufacture because of the welding process. Welded pistons also require that a drilled orifice be provided through the unit for lubrication of the slipper running face. These drilled holes are usually relatively long and small in diameter. Therefore, the drilling process is typically quite difficult and expensive.
Solid pistons also reduce the oil volume. However, solid pistons are much heavier than their hollow counterparts and therefore reduce the speed capability of the hydraulic unit. Similar to welded pistons, solid pistons have a small hole therethrough which requires an expensive drilling operation to ensure lubrication for the slipper running face.
Filling the pistons by pouring a liquid plastic material into them has also been tried. When solidified, the plastic has a bulk modulus greater than that of oil. This method has proven to be costly, and it has been difficult to reliably retain the material within the piston or adhere it to the piston wall. Many plastics do not meet the bulk modulus requirement.
It has been difficult to adapt the conventional "filled" pistons described above to lower-pressure hydraulic units. Thus, the lower-pressure hydraulic units do not get the benefit of the reduced oil volume because they are typically lower-cost units, and the market will not tolerate the additional cost of the non-hollow pistons.
Therefore, a primary objective of the present invention is the provision of a filled piston which is economical to produce and wherein the material which fills the piston is easily secured within the piston.
A further objective of the present invention is the provision of a piston filling having a bulk modulus greater than oil.
A further objective of the present invention is the provision of a filled piston which can be incorporated into low-pressure hydraulic units at a reasonable cost.
A further objective of the present invention is the provision of a piston filling which eliminates the need for secondary operations such as drilling.
A further objective of the present invention is the provision of a lightweight filling for a piston.
A further objective of the present invention is the provision of a lightweight piston filling which can be produced by relatively inexpensive casting or extrusion methods.
A further objective of the present invention is the provision of a filled piston which has an improved structure for retaining the filling therein.
These and other objectives will be apparent from the drawings, as well as from the description and claims which follow.